The present invention relates to a method of balancing an apparatus-produced error signal, which may be large when the apparatus is initially switched on, or activated, but which thereafter changes only slowly with time, particularly an error signal delivered by a measuring sensor or like measuring detector which is independent of the quantity being measured. The invention also relates to a circuit for use when carrying out the method.
The need to be able to balance an error signal which may be initially large but which, in a steady state condition, changes only very slowly with time, is found in many different contexts, one example being measuring sensors and the like which are intended to measure different quantities, these devices normally being incorporated in a measuring bridge.
It is often difficult or impractical to calibrate the measuring bridge prior to a measuring operation, and the bridge is therewith liable to produce an error signal, a so-called offset, even before it is influenced by a quantity to be measured.
In such a case, one approach is to consider all dynamic signals as useful signals. Then, by ac coupling the measuring bridge and utilizing a bottom limit frequency which is sufficiently low for the application concerned the system can be caused to measure only the desired, dynamic changes. Thus, the use of a low limit frequency makes it possible to measure also very slow changes. An ac coupled system is normally obtained by connecting capacitors between the measuring bridge and associated amplifiers.
In this case, the lower limit frequency of the measuring system is set with the aid of the time constant determined by the capacitors and resistors included in the system. One drawback with this solution, however, is that the size of the capacitors required increases with decreasing values of the lower limit frequencies. For example, if a lower limit frequency of 1 Hz is desired and reasonable resistance values are used in the amplifiers, the capacitors will be in the order of magnitude of .mu.F. The problem will naturally increase when requiring a lower limit frequency of 0.01 Hz, for instance. However, it has been possible to solve this problem with the aid of low leakage capacitors and amplifiers with high impedances.
If the intention is to integrate such equipment on a silicon chip, however, completely new problems arise. Admittedly, capacitors and resistors can be mounted on a silicon chip in addition to transistors and diodes. However, the larger the capacitors to be produced, the more useful chip surface is taken-up, and it is also difficult technically to produce capacitors above a certain rating, normally a rating of 10-100 pF. Thus, when practicing present-day techniques, it is extremely difficult to integrate ac coupled amplifiers of low bottom-limit frequencies on a silicon chip.
One alternative to the use of an ac coupled system is to supply constantly a balancing or equalizing signal which will compensate the error signal produced by the measuring sensor. In this regard, if the bridge output signal is returned undamped through a negative feedback loop, the output signal will be constantly zero. However, the useful component of the measurement signal, when it appears, will also be balanced.